Early this year Newsweek magazine published a story with this advice for readers: “Brush your teeth, but the fluoride from your tap may not do much good – and may cause cancer”. The supporting cancer hazard evidence was an animal study sponsored by the government’s National Toxicology Program (Lancet, Feb 3, p282). Since then, the Public Health Service has responded to this and similar news stories by reaffirming its confidence in the safety and effectiveness of fluoridated drinking water. The NTP findings, said the PHS, were too equivocal to conclude otherwise. The headlines stopped.

The scientific debate, however, goes on. On Aug 28 William L. Marcus, chief toxicologist for the Environmental Protection Agency’s drinking water programme, claimed that the original findings of the NTP study showed the cancer hazard from fluoridated drinking water to be greater than the NTP was telling the public. His analysis was presented, without endorsement from the EPA, to the American Chemical Society’s 200th biannual meeting in Washington.

Judging from Dr Marcus’ presentation, this is a case of a disagreement among scientists. The original study was directed from 1985 to 1987 by Dr John D. Toft II, manager of the pathology section at Battelle Memorial Institute in Columbus, Ohio. The Battelle study’s principal finding was the occurrence of an extremely rare liver cancer, hepatocholangiocarcinoma, in male and female mice. In 1989, the NTP asked Experimental Pathology Laboratories, of Sterling, Virginia, to review Battelle’s data. At this point, the liver cancer finding, along with a diagnosis of metaplastic and precancerous cells in the mouths of rats, was downgraded.

The only effect of fluoride that was left after these reclassifications and still another review by a board of pathologists and others was osteosarcoma. Dr Marcus believes the Battelle diagnosis of liver cancers was sound and should have been included in the NTP report. This, he says, would change “the (NTP) equivocal finding… to at least some evidence or clear evidence of carcinogenicity”.

NTP’s failure to emphasize another finding also figured in Dr Marcus’ critique. Three out of four in-vitro tests, he says, proved fluoride to be mutagenic, “supporting the conclusion that fluoride is a probable human carcinogen”. A careful reader can find this information in the text of the report, but the authors make no mention of these data in their conclusions.

As the next step, Dr Marcus wants the EPA to conduct an independent review of the NTP study. The agency, however, wants to wait for the completion of a literature search by a Health and Human Services panel headed by Dr Frank Young, a former Commissioner of the Food and Drug Administration. What may rank with equal importance in the public mind to the possibility of hazard is the question of effectiveness. The National Institute of Dental Research’s latest estimate is that children in fluoridated areas have 18% fewer cavities than those drinking untreated water.

John Yiamouyiannis, a biochemist and antifluoridation leader, has analysed NIDR’s data and says the advantage is 5% at the most. Keeping in mind that a full set of teeth has 128 surfaces, he says, that means the disadvantage for children drinking unfluoridated water will be, on the average, additional decay in less than one of those surfaces.